Update 0142.环形链表II.md

添加 0142.环形链表II.md Java版本

problems/0142.环形链表II.md

@@ -186,6 +186,29 @@ public:
 
 
 Java：
+```java
+public class Solution {
+    public ListNode detectCycle(ListNode head) {
+        ListNode slow = head;
+        ListNode fast = head;
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+            if (slow == fast) {// 有环
+                ListNode index1 = fast;
+                ListNode index2 = head;
+                // 两个指针，从头结点和相遇结点，各走一步，直到相遇，相遇点即为环入口
+                while (index1 != index2) {
+                    index1 = index1.next;
+                    index2 = index2.next;
+                }
+                return index1;
+            }
+        }
+        return null;
+    }
+}
+```
 
 
 Python：